Functional Genomics of Atrial Fibrillation: Understanding Intrinsic and Extrinsic Mechanisms

Abstract

Atrial Fibrillation (AF) is the most common cardiac arrhythmia, affecting over 33 million people worldwide. However, its underlying molecular mechanisms and genetic networks are still poorly understood. Recently, Genome Wide Association Studies (GWAS) have identified several polymorphic variants linked to an increased risk of AF. Almost all such variants are located in non-coding regions, which led us to hypothesize that they could reside in functional elements that interact and regulate genes involved in the origin and development of AF. To test that hypothesis, we have performed in silico screening of AF associated loci toward selecting candidate regulatory elements to study the physical interactions that they establish with neighbouring regions by Circular Chromosome Conformation Capture followed by deep sequencing (4C-seq). From all selected regions, 7q31 locus contains a potential regulatory element inside intron 2 of CAV1 that through chromatin folding could establish long-range interactions with CAV1, TES, MET, and CAPZA2. Deletion of this candidate regulatory element by CRISPR/Cas9 mediated genome-editing in human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) leads to a reduction of CAV1 expression. This finding could be very relevant for AF since CAV1 has a key role on the formation of caveolae, membrane invaginations with multiple roles as holding ion-channels involved in action potential generation. Moreover, we show that in the mouse Cav1 is expressed in the atrial myocardium from early stages of cardiac development up to adulthood. To further study the molecular mechanisms behind AF, we have also used a well characterized sheep model of persistent AF. This model mimics AF progression from paroxysmal episodes to persistent and long-standing persistent AF as it occurs in many patients. Transcriptome analysis of the posterior left atrial wall evidences a strong structural remodelling and suggests that during AF development there could be a remodelling of the innervation of the autonomous nervous system. Additionally, gene expression profile of isolated cardiomyocytes from the left and right atria have revealed that, during AF progression, there is a significant alteration of gene transcription regulation that can lead to cardiomyocyte dedifferentiation. Protein expression profile of those cell populations unveils an overexpression of the subunits of the respiratory chain complexes that can produce an increase in oxidative stress that could promote AF development. Furthermore, we find that changes identified in long-standing persistent sheep (one year in AF) are already present only one week of self-sustained AF. All these results contribute to a better understanding of AF progression.